Direct conversion architecture are very appealing to receiver designers because they require fewer components and therefore are less complex than traditional super-heterodyne receivers. Direct conversion receivers however suffer from a major problem, which is DC-Offset. Direct conversion receives take a radio frequency (RF) signal and translates it too base-band. So if there is any DC-Offset added to the received signal by the mixer (or any other component in the receiver), the DC component of the received signal will be lost because there is no intermediate frequency (IF) filter. Direct conversion receivers are very susceptible to the presence of blocking (interferer) signals that through self-mixing can produce a DC-Offset that is added to the received signal.
In the presence of a time varying DC-Offset, the receiver performance degrades with the amount of degradation depending on the DC-Offset variation and the speed of the variation. That is why solving for time-varying DC-Offset problems is one of the more critical problems to be resolved in a direct conversion receiver. One of the main reasons for the apparition of time-varying DC-Offsets in a direct conversion (also called homodyne) receivers is the presence of TDMA interferes, that through self-mixing or even order non-linearities, can produce a DC-Offset that is added to the received signal. This is something that does not happen with super-heterodyne receivers. Because such an interferer can appear at any time, the DC-Offset component that it will add to the received signal can also appear at any time. As a consequence, it is called a “time-varying” DC-Offset, because it will change depending on the arrival time of the interfering signal.
The DC-Offset problem worsens when the DC-Offset varies with time. Especially when under high AM suppression levels in non-static channel conditions. If one considers that more than one blocking signal can arrive during a single burst of a signal that is to be decoded, one can realize the complexity of the problem at hand. In a related application, entitled “Method for Estimating and Removing a Time-varying DC-Offset”, applicants describe a method for estimating and removing a complex DC-Offset pattern. Although a very good estimate, which allows receivers to pass GSM's AM suppression test, a further improvement on such a DC-Offset estimate or other DC-Offset estimates using other techniques, would be welcome in the art, especially when dealing with multiple blocking signals.